Exemplary embodiments of the present invention relate to a method and an apparatus for determining the integrity risk of an alert limit of a position solution determined with a satellite navigation system.
Integrity is becoming increasingly important for satellite navigation systems, especially for safety-critical applications such as flight navigation based on satellite navigation. One concept for providing integrity in a satellite navigation system is to continuously monitor the satellites from the ground and to transmit data about the monitored satellites, for example, to user systems of the satellite navigation system with the navigation signals output by the satellites. These data can make it possible for a user system to calculate its individual integrity risk and to output a warning or cancel a navigation operation or not even to begin one.
The data on the monitored satellites may be statistical descriptions of properties of the SIS (signals in space) signals emitted from the satellites. In particular, the statistical descriptions may be distributions of SIS errors so that a user system may estimate an integrity risk using this distribution.
For instance, in the future European Galileo satellite navigation system there will be an integrity data stream that makes it possible to signal system or satellite failures to user systems. In particular, faulty satellite signals may be provided with integrity warnings (system warning mechanism). It will also be possible to transmit statistical descriptions of SIS properties with the integrity data stream so that user systems may determine their integrity risk (user integrity concept).
The integrity concept for Galileo and its principles are described in detail in “The Galileo Integrity Concept,” V. Oehler, F. Luongo, J.-P. Boyero, R. Stalford, H. L. Trautenberg, ION GNSS 17th International Technical Meeting of the Satellite Division, Sep. 21-24, 2004, Long Beach, Calif., in published European patent application EP 1 637 899 A1 and granted European patent EP 1 792 196 B1. Helpful details of the Galileo integrity concept shall be explained again in the following to enhance understanding of the present invention.
The Galileo integrity concept provides that the “processing facility” for the ground segment of Galileo makes a prediction about the accuracy of navigation signals. This prediction is a statistical description of the signal error and is called an expected signal error or Signal in Space Accuracy (SISA).
The actual errors of the individual navigation signals transmitted by satellites, or Signal in Space Errors (SISE), are estimated by observations made by the monitoring network of the Galileo system. The estimated errors are called estimated SISE (eSISE).
With respect to the Galileo integrity concept, a satellite is set to not usable via an alert (IF: Integrity Flag) as soon as the estimated signal error (eSISE) for this satellite is greater than an integrity alert threshold (TH: Threshold). Such alerts are transmitted to Galileo's user systems as system warnings.
Since the estimated eSISE for the SISE is an error-prone process, as a rule it is assumed that the distribution of the current SISE about the value of the estimated SISE (eSISE) may be described with a Gaussian distribution with the standard deviation, which is called the Signal In Space Monitoring Accuracy (SISMA). Consequently SISMA represents a statistical description of the accuracy of the estimated eSISE for the SISE for a satellite.
The Galileo integrity concept also provides for transmitting the two statistical descriptions, SISA and SISMA, to user systems that may then use these values to calculate their individual integrity risk in terms of the user integrity concept.
For the Galileo integrity concept and calculating the integrity risk it is now assumed for each satellite that a satellite is in one of the following modes:
Mode 1: The satellite is fault-free and the statistical description of the fault-free satellite is also correct. In the case of Galileo, this means that the fault probability distribution of the satellite is overbounded by the SISA value of Galileo.
Mode 2: The satellite is faulty and with respect to the statistical description of the faulty satellite it is assumed that the satellite has a positive or negative basic fault of the magnitude TH, the entire fault is composed of the basic fault and one additional fault, and in the case of Galileo the probability distribution of the additional fault is overbounded by the SISMA value of Galileo. In the Galileo integrity concept, the basic fault or the integrity alert threshold TH is calculated as follows as a product of a prefactor and the root of the sum of the squares of SISA and SISMA:TH=kpfa·√{square root over (SISA2+SISMA2)}
The prefactor kpfa is determined by the allowed false alert rate.
As a user integrity concept, the calculation of the integrity risk at a so-called alert limit AL proved practical for Galileo (different alert limits HAL and VAL are used for the horizontal and vertical, respectively). It is assumed that either all n satellites that a user system uses are in Mode 1 or all satellites except one are in Mode 1 and the one satellite is only in Mode 2.
The integrity risk is now calculated as the weighted mean of the n+1 possible modes (n modes 2, in which one of the n satellites is assumed to be in Mode 2, and 1 Mode 1, in which all satellites are assumed to be in Mode 1), wherein for simplification the weight for the mode condition in which all satellites are in Mode 1 is frequently assumed to be 1 and the weight for each of the other n conditions in which one satellite is in Mode 2 is assumed to be p_failed. The integrity risk from the other possible conditions of the system is taken into account with a constant additive term in the integrity risk at the alert limit. However, this leads to reduced availability when the SISMA is high.
Exemplary embodiments of the present invention are directed to increasing the availability of a position solution determined with a satellite navigation system with an integrity risk at an alert limit.
In order to increase the availability of a position solution determined with a satellite navigation system having an integrity risk at an alert limit, in accordance with the invention the unweighted contribution to the integrity risk at the alert limit under the assumption that satellite j is faulty is calculated under two different assumptions, and the minimum of the two calculated amounts is used. The first integrity risk at the alert limit is calculated using the classic assumptions that were described in the foregoing that the satellite j is assumed as described by Mode 2. The second integrity risk at the alert limit is calculated as follows: The difference between the position solution with all satellites and the position solution with the satellite j removed from the position solution is deducted from the alert limit and then the integrity risk with the n−1 satellites that are described by Mode 1 is calculated at the reduced alert limit.
One embodiment of the invention relates to a method for determining the integrity risk at an alert limit of a position solution determined with a satellite navigation system, signals received from satellites in the satellite navigation system being processed for determining a position solution and the method having the following steps:
A first integrity risk at the alert limit is calculated under the assumption that one satellite j of the satellites is faulty;
A first position solution is determined with the signals received from all of the satellites;
A second position solution is determined with the signals received from all of the satellites except for the signal received from the satellite j;
A difference between the first and the second position solution is found;
A reduced alert limit is created by subtracting the difference found from the alert limit;
A second integrity risk at the reduced alert limit is calculated with the signals received from all satellites except the signal received from the satellite j; and
The integrity risk at the alert limit is determined using the minimum of the first and second integrity risks.
The integrity risk at the alert limit may be determined using the minimum of the first and second integrity risks as an unweighted contribution in accordance with the user integrity concept of the Galileo satellite navigation system.
Moreover, statistical descriptions provided by the satellite navigation system regarding signal errors of each satellite may be processed in that a basic error is determined therefrom for a faulty satellite, and this basic error is evaluated as faulty for the classification of a satellite.
It is possible to process as statistical descriptions of signal errors for each satellite the expected signal error or the Signal in Space Accuracy (SISA) value for each satellite signal, and, the accuracy of the estimate of the signal error or the Signal in Space Monitoring Accuracy (SISMA) value for each satellite signal, in that for each satellite the basic error TH is calculated according to the following equation:TH=kpfa·√{square root over (SISA2+SISMA2)}wherein the prefactor Kpfa is determined by an allowed false alert rate.
Another embodiment of the invention relates to a computer program with program code for performing all method steps according to the invention and, as described in the foregoing, when the computer program is executed in a computer.
One embodiment of the invention furthermore relates to a non-transitory data carrier on which the program code of the computer program that is executable by a computer is stored as described in the foregoing.
In another embodiment, the invention relates to an apparatus for determining the integrity risk at an alert limit of a position solution determined with a satellite navigation system, the apparatus being embodied for processing signals that are received from satellites for the satellite navigation system for determining a position solution and the apparatus having the following:
first means for calculating a first integrity risk at the alert limit under the assumption that one satellite j of the satellites is faulty;
second means for determining a first position solution with the signals received from all satellites;
third means for determining a second position solution with the signals received from all satellites except the signal received from the satellite j;
fourth means for determining a difference between the first and the second position solution;
fifth means for forming a reduced alert limit by subtracting the determined difference from the alert limit;
sixth means for calculating a second integrity risk at the reduced alert limit with the signals received from all satellites except the signal received from the satellite j; and,
seventh means for determining the integrity risk at the alert limit using the minimum of the first and second integrity risks.
The first through seventh means may be implemented, at least in part, by a programmable processor and a program that is stored in a memory and that is for configuring the programmable processor for performing one or a plurality of steps of a method in accordance with the invention and as described in the foregoing.
Finally, one embodiment of the invention relates to a position determination apparatus for determining a position using signals from a satellite navigation system and for determining an integrity risk at an alert limit for the determined position, the position determination apparatus having an apparatus for determining the integrity risk at an alert limit of a position solution that is determined with a satellite navigation system in accordance with the invention and as is described in the foregoing and having an output means for outputting a determined integrity risk.
The output means may especially have one or a plurality of the following means: a display unit, a data output interface, and/or an audio output unit.
Additional advantages and application options for the present invention result from the following description in conjunction with the exemplary embodiments depicted in the drawings.
The terms used in the list of reference numbers included at the end of this specification and the associated reference numbers are used in the description, claims, abstract, and drawings.
In the following specification, identical, functionally equivalent, and functionally associated elements may be provided with the same reference number. Absolute values are provided only as examples in the following and shall not be construed to be limiting for the invention.